1. Field of the Invention
The present invention relates to a power circuit and a radiocommunication apparatus using the same. The present invention, more particularly, relates to a power circuit to supply power by stepping up the output voltage of a battery and a radiocommunication apparatus using the power circuit.
2. Background of the Invention
In mobile communication equipment such as a portable telephone which a user can carry and use for communication, its small size and light weight is an important factor for convenience of users.
Electronic circuits incorporated in radiocommunication apparatuses are generally formed with semiconductor devices so that the apparatus may be small sized, light weight, and power saving. Of the parts mounted on a radiocommunication apparatus, one main element which is particularly bulky and weighty is a battery as the power source to operate electronic circuits in the radiocommunication apparatus.
While electronic circuits in a radio communication apparatus are designed to achieve energy saving, it is required for the apparatus to amplify its transmitted signal to a certain level in order that the transmitted wave output, i.e., the transmitted distance, is kept above a certain value. Therefore, the power amplifier circuit used for the amplification is operated by a voltage higher than the operating voltage of the circuits for processing transmitted and received signals.
Thus, in a radiocommunication apparatus, there are incorporated circuits operated by at least two kinds of voltages, i.e., signal processing circuits which are constituted of a microprocessor, or DSP, DRAM, and the like, operated by a low voltage, and a power amplifier circuit operated by a higher voltage than that for the signal processing circuits. Therefore, it is required for the power circuit to supply at least two kinds of voltages.
The power consumed in the power amplifier circuit is larger than the power consumed in the signal processing circuits. Hence, the battery capacity is generally specified by the transmitted wave power.
In order to increase the transmission distance, i.e., transmitted wave power, it is necessary to increase the battery capacity. However, the increase in the battery capacity contradicts the requirement of the radiocommunication apparatus to be light and portable.
As a method to overcome the above mentioned problem, there is a method for example disclosed in the gazette of Japanese Laid-open Patent Publication No. Sho 61-144131. The method disclosed in the gazette of Japanese Laid-open Patent Publication No. Sho 61-144131 is based on the fact that time division multiplex communication is generally practiced in the mobile communication and it is proposed therein that power of a battery be stored in a capacitor at the timing (during the period) when the radiocommunication apparatus performs no transmission and the stored power on the capacitor be supplied to a power amplifier circuit at the timing when the radiocommunication apparatus performs transmission.
The time division multiplex communication for example is such a communication method that divides one frame into a plurality of slots and allots each slot to a call to thereby make it possible for a plurality of calls to be made at the same time.
As a system for the time division multiplexing, for example TDMA system, or TDD system, is known.
According to the above mentioned method, such merits can be obtained so that power saving in the power amplifier circuit can be achieved because the time required for operating the power amplifier circuit can be shortened and, since the power is supplied to the power amplifier circuit from the power stored on the capacitor, a large current free from the internal resistance of the battery can be supplied. Namely, even if the battery capacity is not large, a radiocommunication apparatus capable of transmitting signals over a long distance can be structured.
However, there still are some drawbacks in the method disclosed in the gazette of Japanese Laid-open Patent Publication No. Sho 61-144131.
A battery with a low output voltage is generally small and economical. In the method disclosed in the gazette of Japanese Laid-open Patent Publication No. Sho 61-144131, however, it is required that a battery whose output voltage is corresponding to the operating voltage of the power amplifier circuit should be used. For example, supposing that the operating voltage of the power amplifier circuit is 7 V and normally used nickel-cadmium (Ni--Cd) cells with a voltage of 1.3 V or so per cell are used as the battery, it is required that six cells of the nickel-cadmium cells should be used. Hence, the battery comes to occupy a large space (becomes bulky) and this contradicts the requirement of the radiocommunication apparatus to be smaller in size.
There is proposed a method in which a low-voltage battery is used and it is adapted such that the signal processing circuits are supplied with the battery voltage and the power amplifier circuit is supplied with a high voltage obtained by stepping up the battery voltage by means of a DC/DC converter or the like.
The above method, however, has not been applied to a radiocommunication apparatus on a time division multiplex system. Since the operation of the power supply at the timing corresponding to the time division multiplexing is not considered in this method, it is still encountering the difficulties of the battery capacity and the bulkiness of the DC/DC converter. In addition, since the DC/DC converter does not have 100% efficiency, efficient power supply cannot be achieved with it alone.
Therefore, such a problem is encountered in this method that the span of life of the battery becomes short.
Of course, an attempt is being made to achieve power saving of the power amplifier circuit itself.
For example, a gallium-arsenide (Ga--As) semiconductor device is suitable for operation at low voltage and applicable, as a power amplifier circuit element, to a high-frequency power amplifier circuit (RF power amplifier), which is being developed for use in transmission with portable mobile transmission equipment, such as a portable telephone. Its standard power-supply voltage is 5.8 V or 5.0 V. Namely, while a voltage of 7 V or so was earlier required for the power-supply voltage, it has been lowered to around 5.8 to 5.0 V.
While the operating voltage of signal processing circuits earlier was around 5 V, it has been lowered to around 2.7 to 3.3 V owing to the development in the semiconductor circuit technology.
The inventors of the present application have already proposed a power supplying method suitable for operating electronic circuits in a radiocommunication apparatus on a time division multiplexing system having at least two kinds of electronic circuits operating at two kinds of voltages as described above for example in the gazette of Japanese Laid-open Patent Publication No. Hei 4-315320.
In this method, signal processing circuits other than the power amplifier circuit are arranged to be operated by a low voltage around 3 V and a battery whose ultimate voltage is around the operating voltage of the signal processing circuits is mounted thereon so that the signal processing circuits can be operated directly by the output voltage of the battery. On the other hand, as the driving (operating) voltage of the power amplifier circuit, the output voltage of the battery is stepped up to the operating voltage of the power amplifier circuit, for example 5 V or above, using a DC/DC converter. Further, the power of which the voltage is stepped up is stored in a small capacitor while the apparatus is not transmitting a signal and, while the apparatus is transmitting a signal, the power stored on the capacitor is supplied to the power amplifier circuit to operate it preferably through a regulator, which adjusts the voltage level of the supplied power.
As a result, a meritorious effect was obtained that the power supply to the signal processing circuits operating at low voltage and the power supply from the battery to the power amplifier circuit operating at higher voltage can be efficiently carried out.
Since what was disclosed in the gazette of Japanese Laid-open Patent Publication No. Hei 4-315320 was the basic circuit configuration and operation of the circuit, there were not exemplified particular types of batteries, but use of a Nickel-Cadmium (Ni--Cd) battery was considered suitable for higher efficiency, in view of its small change in the discharge drooping characteristic, and practical according to the art at the time of the application.
However, since the output voltage of the Nickel-Cadmium battery is as low as 1.2 V per cell as shown in FIG. 1, three Ni--Cd cells must be used even if the signal processing circuits is arranged to be operated by a low voltage of 3 V. Thus, there has still been a limit in decreasing the size of the radiocommunication apparatus.
Further, the use of Nickel-Cadmium batteries has come to be controlled on account of their causing environmental pollution.
On the other hand, a lithium ion cell has come to be taken note of as a pollution-free cell and as the constituent of a battery having a high output voltage (large battery capacity) per cell, its full-charged voltage being around 4.3 V and its ultimate voltage around 2.7 V, for example, as shown in FIG. 1.
If the ultimate voltage is around 2.7 V, the signal processing circuit driven by the voltage from 2.7 to 3.3 V can be operated using only one cell of the lithium ion cell and this contributes to realization of a small radiocommunication apparatus. Therefore, the lithium ion cell is preferred to the nickel-cadmium cell as the cell to be mounted on the radiocommunication apparatus.
Discharge drooping characteristics of the nickel-cadmium (Ni--Cd) cell and the lithium ion cell are shown in FIG. 1.
However, in the discharge drooping characteristic of the lithium ion cell, the discharging curve linearly lowers from the full-charged voltage to the ultimate voltage producing a large potential difference, from 100% to 60-65%.
As a result, there arises a problem that energy at the portion of the area a in FIG. 2 is not used effectively. This problem becomes remarkable particularly when a power-supply regulator with its output voltage close to the ultimate voltage is inserted between the lithium ion cell and the signal processing circuit.
This is because the output voltage of the power-supply regulator is constantly set to the ultimate voltage even when the output voltage of the cell is above the ultimate voltage, and then, the power obtained from the voltage exceeding the ultimate voltage multiplied by the output current is dissipated in the power-supply regulator.
Details of it will be described below.
The energy in the area (b) shown in FIG. 2 is the energy effectively supplied from the cell to each circuit.
Energy utilization efficient .eta. is given by the expression (1) below. EQU .eta.=energy in area (b) in FIG. 2/ [energy in area (b) in FIG. 2+energy in area (a) in FIG. 2] (1)
When it is assumed that an ideal power-supply regulator whose voltage drop is 0 V is used with its output set to the ultimate voltage of the nickel-cadmium cell to supply power to each circuit in the radiocommunication apparatus, the efficiency .eta. becomes approximately 95% because the change in the voltage drooping characteristic is small.
On the other hand, the efficiency .eta. with the lithium ion cell becomes as low as approximately 77% because the change in the discharge drooping characteristic is large and the energy at the portion in the area (b) in FIG. 2 is wasted.
Thus, when a lithium ion cell is used, it provides a greater energy density, but, because its discharge drooping characteristic has a large change in the output voltage, there arises the problem of its efficiency .eta. becoming lower than the nickel-cadmium cell when it is used for supplying power to each circuit of the radiocommunication apparatus through a power-supply regulator.
These days, the driving voltage of a large capacity DRAM is becoming as low as the level of 1 V.
The driving voltage of the signal processing circuit formed with a microprocessor is becoming still lower. On the other hand, the driving voltage of a transmitting power amplifier circuit is still high and power consumption therein is great. Hence, there is a tendency that the difference between the driving voltages of the power amplifier circuit and the signal processing circuit is becoming greater and, hence, the above described problem is becoming still more serious.